The thermal denaturation of bovine albumin (BA) was originally studied by Privalov at low ionic strength and at Ph 7.0 exclusively [Mol. Biol, (Mosc.) 19, 1072-1078 (1985)]. Privalov was able to adequately deconvolute his thermograms for BA and a tryptic fragment into sums of two-state envelopes, which correspond to the unfolding of the constitutive domains; albumin is comprised of three major domains. Initially we investigated the effect of Ph at low ionic strength on the thermal denaturation of human albumin (HA) from Ph 3.7-11-0 and generally found two major peaks although at pH's 4.0, 4.3, and 7.5, single peaks obtained. The denaturation temperatures of these peaks showed a marked Ph dependence. This effect of Ph on the thermal unfolding behavior is reversible since at Ph 5.65 the two major denaturation peaks were unchanged by Ph 5.65 -> 3.7 -> 5.65. In addition, over the range Ph 5.5-1-.0, there was a smaller endotherm at higher temperature, which was ascribed to a small amount of A stabilized through the binding of a residual amount of long-chain fatty acid. At Ph 5.5 the thermal denaturation process was reversible for the first endotherm but not the second. Due to the relative facility in obtaining proteolytic fragments of BA, we opted to study the unfolding of this albumin. The effect of chloride binding was studied at Ph 4.0 since the two major peaks are most separated here in comparison with the separation at Ph-s 3.5 and 5.3, the other pH's studied; in the range of 30-50 Mm C1-, which is where significant C1- binding begins, the peaks begin to merge. From BA with the free sulfhydryl blocked by reaction with L-cystine (cys-BA), we prepared two complementary pepsin fragments, PA (307-581) and PB (1-3060, which associate at alkaline Ph to form a complex with some properties similar to those of native BA. The thermograms of PA+PB at Ph 4.0 (not associated) and Ph 8.6 (associated) both could be approximated as the sum of those of the isolated fragments at the corresponding pH's thereby suggesting little interaction between domains in the complex at Ph 8.6. However, the most striking effect was the difference in denaturation between BA and cys-BA at Ph 8.6; the thermogram for unmodified BA was comprised of three major peaks whereas that for cys-BA consisted of only two peaks.